1. Field of the Invention
This invention relates to method and apparatus for simultaneously bonding a plurality of lead frames to a plurality of planar articles, and more particularly, to method and apparatus for simultaneously mass thermocompression bonding a plurality of lead frames to a plurality of planar articles.
2. Description of the Prior Art
The beam-leaded semiconductor device is a recent development which comprises a semiconductor element including a chip or die of semiconductor material within which is fabricated, by known planar processing techniques, either an individual semiconductor component, such as a transistor, or a plurality of components suitably interconnected to provide a monolithic integrated circuit network. Formed on one of the major surfaces of the die, during fabrication of the element, are a plurality of beam leads, primarily of gold, which are electrically connected to appropriate regions on the semiconductor die and extend outward beyond the edges of the die in an array parallel to the major surface of the die. The beam leads can be formed in any known manner, such as, for example, by plating techniques or by a film strip process as disclosed, for example, in U.S. Pat. No. 3,777,365, issued to C. W. Umbaugh on Dec. 11, 1973.
The beam leads provide a means for making contact between the die and the metallized portions of a miniceramic substrate, a substrate of a circuit board, or a device enclosure on which the semiconductor element is mounted. The semiconductor device is placed on the surface of the substrate with the beam leads in alignment with the metallized areas of the substrate. The beam leads are then bonded directly to the metallized areas as by thermal compression bonding, ultrasonic bonding, or welding. Thus, the tedious, error-prone, time-consuming prior art process of manually bonding lead wires between semiconductor dies and metallized areas of substrates is eliminated. See, for example, U.S. Pat. No. 3,627,190, issued to H. J. Ramsey on Dec. 14, 1971 which discloses apparatus for the thermocompression bonding of a beam-leaded device to a substrate. It is also known to simultaneously thermocompression bond a plurality of beam-leaded devices to a similar plurality of metallized patterns on one side of a single ceramic substrate. In this regard see, for example, U.S. Pat. No. 3,699,640, issued to B. H. Cranston et al. on Oct. 24, 1972 which discloses method and apparatus for compliant bonding beam-leaded semiconductor devices to a substrate. Once the beam-leaded devices have been bonded to the array of metallized patterns on a substrate, the substrate can be machined to separate the plurality of bonded devices and metallized patterns into a similar plurality of miniceramic substrates having the metallized bonding sites adjacent the edges thereof. The bonding sites on each miniceramic substrate can then be bonded to a lead frame for subsequent interconnection to other circuits.
It is known to thermocompression bond the leads of individual lead frames to separate substrates in rapid succession for increased productivity. The most widely known method for effecting the thermocompression bond is to align both the lead frame and the bonding sites on the substrate with each other and with the elements of a bonding station, and then to apply a heated thermode onto the leads of the lead frame with sufficient compressive force and temperature to effect the bonds. It is also known to transfer the necessary heat through the substrate and to the bonding sites for the thermocompression bond, but under such conditions there has been a tendency towards increased cracking in the substrates unless extreme care and control are used in the bonding process. It is for this reason that the application of the heated thermode to the leads of the lead frame is preferred.
Attempts at applying known methods and apparatus to accomplish the simultaneous thermocompression bonding of a plurality of lead frames to a similar plurality of substrates has resulted in many problems which have tended to discourage further development of a production type mass bonder. The problems encountered related, inter alia, to the fact that the details on the heated thermode engaging the leads, which are referenced to the bonding sites on the substrates, must be accurately placed on the leads to effect a proper bond at each bonding site. The details on the thermode, and the thermode itself, however, expand as the thermode is heated to a particular bonding temperature. It, therefore, became necessary to precalculate the amount of thermal expansion that would take place for a particular thermode for a predetermined temperature and from such calculations accordingly machine the cold thermode. If the bonding temperature had to be sufficiently changed, either higher or lower, it might become necessary to machine a new thermode. An alternative arrangement would be to provide an array of separately heated bonding tips capable of being simultaneously activated. Such arrangement, however, produced, inter alia, other problems such as (a) the necessity to precalculate the dimensions of the pistons, sleeves, and other elements which activate and control the movement of each bonding tip in order to compensate for expansion from the heat conveyed to each of the elements in accordance with its relative position in the array; (b) the ability to individually control the plurality of heating units in a manner to achieve the same temperature at each bonding tip in the array; and (c) to provide a device which simultaneously activates the heated bonding tips and applies an equal predetermined pressure to all bonding tips while the device is being subjected to an elevated temperature from the heating elements associated with each bonding tip. All of the above problems, of course, become more insurmountable as the number of lead frames and substrates to be simultaneously bonded increases.